1. FIELD OF THE INVENTION
This invention relates to improvements in a field emission electron gun, and more particularly to improvements in a power supply circuit thereof.
2. DESCRIPTION OF THE PRIOR ART
When a high negative voltage is applied to a needle-shaped electrode (cathode) whose tip is very sharp, electrons are emitted from a field crowding part of the tip even in case where the needle-shaped electrode is at the normal temperature. As is well known, an electron gun fabricated on the basis of such principle is called the field emission electron gun.
As compared with the case of a conventional thermionic gun, the field emission electron gun has the feature that an electron beam emitted therefrom has a high electron current density, so the diameter of the electron beam can be made very small. In recent years, therefore, the field emission electron gun has come into frequent use as the electron source of an electron beam instrument such as an electron microscope.
However, whereas the electron current of the conventional thermionic gun is almost invariable the lapse of time, the field emission electron current exhibits a unique time variation as shown by a curve a in FIG. 1. More specifically, the value of emission current changes abruptly from I.sub.o to I.sub.1 during the period between a time T.sub.o at which the field emission begins operation and a time T.sub.1 (usually, the period is called the initial unstable region). After the time T.sub.1, the emission current exhibits almost no decrease and proceeds at a substantially constant value, whereupon it increases gradually and reaches a value I.sub.2 at a time T.sub.2 (usually, this period is called the stable region). It is in the stable region that the field emission electron gun can be stably used. After the time T.sub.2, the emission current increases while fluctuating (usually, this period is called the terminal unstable region). In particular, when a time T.sub.3 elapses, greater fluctuations are attended with, and if the field emission is still continued, the field emission cathode will be damaged by an overcurrent or electric discharge. It is accordingly necessary to cease the field emission at this time and clean and reproduce the field emission cathode by, for example, flashing it when the time T.sub.2 has elapsed. When the field emission cathode is cleaned, it is reproduced so that the value of emission current may follow substantially the same time variation as in the above again. The cause for the fusion of the field emission cathode is that the overcurrent flows through the electron source. It is therefore apparent that the value of emission current must not exceed the allowable limit current value of the electron source, as indicated by I.sub.m in FIG. 1, throughout a period from the beginning to the end of the field emission. In other words, a high voltage to be applied to the field emission electron gun need be controlled so that the value of emission current may not exceed the allowable limit value I.sub.m throughout the period of the field emission.
For the above reason, in applying the high voltage to the field emission cathode, the operation has hitherto followed the steps described below. First of all, a sufficiently low voltage corresponding to an electron current of desired value is applied. Then, the electron current of desired value is emitted at the initial stage, but the emission current begins to decrease immediately. When the emission current has decreased to some extent, a voltage being somewhat higher than the first voltage is applied. While repeating such steps, the applied voltage is gradually raised, and the desired value of emission current (I.sub.1) is finally attained stably. In FIG. 2, the states of variations of the value of the applied voltage (V) to the field emission cathode and the field emission current (I) with the lapse of the time (T) in the process of the operation are respectively shown by a curve b and a curve c.
Since, however, such operation is manually executed in the prior art, the job is highly complicated. Moreover, it is still feared that the cathode will be damaged by erroneously applying an excess voltage.